Interference is a defining feature of both quantum and classical theories of light enabling precise measurements of a wide range of physical quantities including length and time. Quantum metrology exploits fundamental differences between these theories for new measurement techniques and enhanced precision. Advantages stem from several phenomena associated with quantum interferometers, including non-local interference, phase-insensitive interference, phase super-resolution and super-sensitivity, and automatic dispersion cancellation.
Arguably, the best known example of quantum interference was demonstrated by Hong, Ou, and Mandel (“HOM”). FIG. 1 illustrates a HOM interferometer in accordance with the prior art. HOM interference is now considered central to optical quantum technologies, including quantum teleportation and linear-optical quantum computing. Several characteristics distinguish HOM from classical interference, such as Michelson's or Young's. The HOM signal stems from pairs of interfering photons and manifests as a dip in the rate of coincident photon detections spanning the coherence length of the light, as opposed to classical wavelength fringes. It is therefore inherently robust against path length fluctuations. If the photons are entangled, the visibility and width of the HOM interferogram is typically insensitive to loss and dispersion. Furthermore, HOM interferometers typically achieve higher resolution than classical interferometers using the same bandwidth. These features are ideal for precision optical path measurements of dispersive and lossy materials, implemented by placing the sample in one interferometer arm and measuring the delay required to restore interference. Unfortunately, quantum interferometers require entangled states that are practically difficult to create, manipulate, and detect, especially compared with robust, intense classical states.
Optical coherence tomography (OCT) is a non-invasive imaging technique using low-coherence interferometry to produce depth profiles of a sample. OCT has found many biomedical applications including diagnosis of ocular diseases or detection of early-stage cancer. Axial resolution in OCT is typically ultimately limited by the coherence length of the light source and can be less than 1 μm for very broadband sources. This resolution is typically hindered by material dispersion which serves to both broaden features in the interferograms and reduce contrast. A quantum version of optical coherence tomography (QOCT) has been shown to harness the advantages of HOM interference. QOCT combines the idea of HOM interference with a standard time-domain OCT system to harness the advantages of HOM interference. QOCT techniques have not found widespread application because they suffer from the difficulties of working with entangled photons, such as expensive, complex experimental setups and low signal levels. QOCT, which replaces white light interference (WLI) with a HOM interferometer based on frequency-entangled photon pairs, automatically cancels all even orders of dispersion (including the most significant, group-velocity dispersion) in the resulting interferogram, allows for dispersion cancellation to be “blind” (i.e. requiring no a priori knowledge of the material properties), is phase insensitive, has better resolution than WLI with the same bandwidth, and provides an interference visibility that is insensitive to unbalanced loss. Unfortunately, the HOM interferometer utilized in QOCT is based on entangled photon pairs and the costs, in terms of speed, and specialized & expensive equipment, have limited its widespread adoption. Other techniques for blind dispersion compensation without entanglement have been proposed or demonstrated, but they require unavailable technology or significant numerical post-processing and do not have the other properties of HOM interference.
There are several other techniques which have been used to cancel dispersion in WLI/OCT which each fail to provide a fully beneficial result, including compensating dispersion (which only approximates cancellation and even then is useful only for dispersion cancellation at certain depths of the sample), numerical algorithms (which require a priori knowledge of characteristics of the sample), use of broadband modulators and multipass interferometry (which are very difficult to implement), use of physical assumptions about the material (which also require a priori knowledge of characteristics of the sample), and white-light spectral interference in conjunction with computing a correlation function (which require a large amount of data to be taken and a substantial numerical post-processing). Other techniques require wavelength path stability such that the interference visibility falls precipitously with loss and is limited to 50% of that possible with the HOM effect. Alternatively, background-free autocorrelation of transform-limited pulses, recently used for OCT, exhibits enhanced resolution, phase insensitivity, and robustness against loss, but notably not automatic dispersion cancellation. Other phase-insensitive classical interferometers achieve their phase insensitivity by ensuring that the interfering paths travel through common optics, or even common spatial paths. They are therefore incapable of measuring delays through interference since the relative path lengths cannot be changed.
Therefore, what is needed is an interferometer that does not require entangled photons yet achieves all of the benefits of an HOM interferometer, including: phase-insensitive interference, high interference contrast, automatic dispersion cancellation, and insensitivity to loss. What is also needed is such an interferometer that can achieve these features without requiring a priori knowledge of the nature of the material or extensive numerical post-processing of data.